The present invention relates to the general field of undersea pipes resting on the sea bottom and providing a connection between undersea wells for producing hydrocarbons, in particular oil and gas, and a surface installation, e.g. a floating, production, storage, and off-loading unit.
In a given off-shore hydrocarbon production field, it is common practice to work a plurality of wells that may be spaced apart from one another by several kilometers or even tens of kilometers. The fluids coming from these various wells need to be collected by undersea pipes resting on the sea bottom and transferred via bottom-to-surface connection pipes to a surface installation, e.g. a ship or a shore collection point for collecting and storing them (and possibly processing them).
Furthermore, because the fluids (oil and gas) from the undersea hydrocarbon production wells are extracted at great depth in the subsoil, they leave the wells at a temperature that is relatively high at the level of the sea bottom (typically of the order of 70° C.). Since sea water is generally cold, particularly at great depths where it is typically at 4° C., if no arrangements are made for conserving the temperature of the fluids leaving production wells, they will cool progressively while traveling along the kilometers of undersea pipes. Unfortunately, those fluids contain various chemical compounds for which cooling leads to phenomena appearing that are impediments to maintaining good flow conditions.
Thus, molecules of gas, in particular of methane, combine with molecules of water to form hydrate crystals under certain conditions of pressure and temperature. These crystals can stick to the walls, where they collect together and lead to the formation of plugs capable of blocking the undersea pipe. Likewise, the solubility in oil of high molecular weight compounds, such as paraffins or asphaltenes, decreases with lowering temperature, thereby giving rise to solid deposits that are likewise capable of blocking the undersea pipe.
One of the known solutions for attempting to remedy those problems consists in thermally insulating undersea pipes in order to conserve as much as possible the initial heat of the fluids being transported. Several technologies are available for this purpose, including for example making use of coaxial pipes comprising an inner pipe conveying the fluids and an outer pipe coaxial with the inner pipe and in contact with the undersea medium, the annular space between the inner and outer pipes being filled with a thermally insulating material or else being evacuated.
Nevertheless, that solution of thermally insulating undersea pipes can at best only slow down the inevitable cooling of the fluids being transported. In particular, if the distance to be traveled at the bottom of the water is too great, or if the flow rate of the fluid is slowed or even stopped for a certain length of time, then the temperature of the fluids can drop below a critical threshold.
Another known solution for avoiding the formation of plugs in undersea pipes consists in cleaning them frequently by causing scrapers (also known as “pigs”) to travel inside them and remove any deposits from the walls.
Nevertheless, in order to avoid any risk of a scraper jamming in the pipe, scraping an undersea pipe can at best eliminate only deposits that are of relatively modest sizes. Furthermore, in spite of having recourse to scraping, that solution still requires the pipes to be thermally insulated in order to limit as much as possible the formation of solid deposits inside them.
Another known solution consists in heating undersea pipes over their entire length by one or more electric cables that are wound around the pipes to heat them by the Joule effect. That solution is referred to as heat tracing and serves to keep the fluids transported in undersea pipes at a temperature higher than a critical threshold over their entire path from the production well to the surface installation.
That solution presents manifest problems associated with installing such electric heater cables over the entire length of the undersea pipes, with the high costs that are involved in terms of installation. Furthermore, heat tracing is based on continuity of the installation all along the undersea pipes. Unfortunately, if such continuity should be interrupted for any reason at a particular location in the pipes, then the entire installation is put out of service. This constraint thus makes it necessary to consider that type of heating only during so-called “preservation” stages for preserving the transported fluids, and not for operational stages.